Due to current (amperage) limits of batteries, most laptops have VERY small current caps on USB ports. Some desktops can have 1-2 amps per USB port, but laptops are often limited to 300 to 500 milliamps. This is sufficient for most memory sticks/pen drives, but many USB devices need much more power than that. For example, external USB-HDD cases for 2.5 inch hard drives will need 100-200 mA ABOVE the rating of the HDD, which is usually 500-750 mA. Few laptops can provide that much current, or will initially provide enough to spin up, but get errors or fail to mount due to lack of power. If your USB device has a secondary power jack, use it with a 5 volt AC power supply with the correct jack polarity and current rating. You can also try a "double harness" USB cable, which plugs into the laptop with TWO USB jacks, and into the USB HDD case with one (the third) jack. This allows TWO USB ports to "double" the available current for the USB device. Best to get the specs on your laptop, see what your USB current limits are, before doing this. If each port is limited to 300 mA and your HDD is 500 mA, you will be OK, but if the HDD is 750 mA, the total of 600 mA is not going to spin up the drive.
Dec 03, 2007 |